Transistor having replacement metal gate and process for fabricating the same

ABSTRACT

A transistor is fabricated by removing a polysilicon gate over a doped region of a substrate and forming a mask layer over the substrate such that the doped region is exposed through a hole within the mask layer. An interfacial layer is deposited on top and side surfaces of the mask layer and on a top surface of the doped region. A layer adapted to reduce a threshold voltage of the transistor and/or reduce a thickness of an inversion layer of the transistor is deposited on the interfacial layer. The layer includes metal, such as aluminum or lanthanum, which diffuses into the interfacial layer, and also includes oxide, such as hafnium oxide. A conductive plug, such as a metal plug, is formed within the hole of the mask layer. The interfacial layer, the layer on the interfacial layer, and the conductive plug are a replacement gate of the transistor.

RELATED APPLICATIONS

The present patent application is a continuation of the previously filedpatent application having the same title, filed on Sep. 11, 2010, andassigned patent application Ser. No. 12/880,085.

BACKGROUND

Transistors are semiconductor devices that are commonly found in a widevariety of integrated circuits. A transistor is basically a switch. Whena voltage is applied to a gate of the transistor that is greater than athreshold voltage, the switch is turned on, and current flows throughthe transistor. When the voltage at the gate is less than the thresholdvoltage, the switch is off, and current does not flow through thetransistor.

Traditionally, the gates of transistors have been polysilicon gates.Transistors having polysilicon gates are relatively easy to fabricate,and the operating effects of transistors having polysilicon gates arewell known. However, as power consumption and operating speeds ofintegrated circuits that include transistors are optimized, morerecently the gates of transistors have been metal gates.

A transistor having a metal gate can be fabricated in two general ways.A polysilicon gate can first be fabricated, and then replaced with ametal gate during subsequent semiconductor processing. This approach isknown as a “gate last” approach, and the metal gate is considered adamascene or replacement gate, insofar as the metal gate replaces apolysilicon gate. A second approach fabricates the metal gate withoutfirst fabricating a polysilicon gate, and is known as a “gate first”approach.

BRIEF SUMMARY

A method of an embodiment of the invention is for fabricating atransistor. The method removes a polysilicon gate over a doped region ofa substrate and forms a mask layer over the substrate such that thedoped region is exposed through a hole within the mask layer. The methoddeposits an interfacial layer on top and side surfaces of the mask layerand on a top surface of the doped region exposed through the hole. Themethod deposits a layer on the interfacial layer adapted to one or moreof: reducing a threshold voltage of the transistor and reducing athickness of an inversion layer of the transistor. The method forms aconductive plug within the hole of the mask layer. The interfaciallayer, the layer on the interfacial layer, and the conductive plugtogether are a replacement gate of the transistor.

A method of another embodiment of the invention is also for fabricatinga transistor. The method removes a first polysilicon gate over a p-dopedregion of a substrate, removes a second polysilicon gate over an n-dopedregion of the substrate, and forms a mask layer over the substrate suchthat the p-doped region and the n-doped region are exposed through holeswithin the mask layer. The method covers the hole through which then-doped region is exposed within the mask layer with a first temporarylayer. While the hole through which the n-doped region is exposed withinthe mask layer is covered with the first temporary layer, the methoddeposits a first interfacial layer on top and side surfaces of the masklayer and on a top surface of the p-doped region exposed through thehole. While the hole through which the n-doped region is exposed withinthe mask layer is covered with the first temporary layer, the methodalso deposits a first layer on the first interfacial layer adapted toone or more of: reducing a threshold voltage of the transistor andreducing a thickness of an inversion layer of the transistor. The methodremoves the first temporary layer, and covers the hole through which thep-doped region is exposed within the mask with a second temporary layer.

While the hole through which the p-doped region is exposed within themask layer is covered with the second temporary layer, the methoddeposits a second interfacial layer on top and side surfaces of the masklayer and on a top surface of the n-doped region exposed through thehole. While the hole through which the p-doped region is exposed withinthe mask layer is covered with the second temporary layer, the methodalso deposits a second layer on the second interfacial layer adapted toone or more of: reducing the threshold voltage of the transistor andreducing the thickness of the inversion layer of the transistor, thesecond layer being different than the first layer. The method removesthe second temporary layer, and forms a conductive plug within each holeof the mask layer. The first interfacial layer, the first layer on thefirst interfacial layer, and the conductive plug within the hole throughwhich the p-doped region is exposed are a first replacement gate for thep-doped region of the transistor. The second interfacial layer, thesecond layer on the second interfacial layer, and the conductive plugwithin the hole through which the n-doped region is exposed are a secondreplacement gate for the n-doped region of the transistor.

A method of still another embodiment of the invention is also forforming a transistor. The method forms vertical spacers on side surfacesof a polysilicon gate over the doped region. The method replaces aportion of the doped region at a side of each vertical spacer beginningat the top surface of the doped region with silicide. The method forms amask layer over the substrate and over the polysilicon gate, the masklayer including nitride. The method etches the mask layer to expose thepolysilicon gate, and removes the polysilicon gate. The method depositsan interfacial layer on top and side surfaces of the mask layer and on atop surface of the doped region exposed through the hole. The methoddeposits a layer on the interfacial layer adapted to one or more of:reducing a threshold voltage of the transistor and reducing a thicknessof an inversion layer of the transistor. The method deposits a workfunction metal within the hole of the mask layer, and deposits a secondmetal over the work function metal within the hole of the mask layer.The work function metal and the second metal are a conductive plug. Theinterfacial layer, the layer on the interfacial layer, and theconductive plug together are a replacement gate of the transistor.

A semiconductor transistor device of an embodiment of the inventionincludes a substrate, a doped region within and exposed through thesubstrate, a mask layer over the substrate, and a replacement metal gatewithin the mask layer and over the doped region. The replacement metalgate includes an interfacial layer adjacent to side surfaces of the masklayer and to a top surface of the doped region. The replacement metalgate also includes a layer at least partially diffused into theinterfacial layer, to one or more of: reduce a threshold voltage of thetransistor and reduce a thickness of an inversion layer of thesemiconductor transistor device.

A semiconductor transistor device of another embodiment of the inventionincludes a substrate, a p-doped region within and exposed through thesubstrate, an n-doped region within and exposed through the substrate, amask layer over the substrate. The semiconductor transistor device alsoincludes a first replacement metal gate within the mask layer and overthe p-doped region, and a second replacement metal gate within the masklayer and over the n-doped region. The first replacement metal gateincludes a first interfacial layer adjacent to side surfaces of the masklayer and to a top surface of the p-doped region, and a first layer atleast partially diffused into the first interfacial layer, to one ormore of: reduce a threshold voltage of the transistor and reduce athickness of an inversion layer of the semiconductor transistor device.The second replacement metal gate includes a second interfacial layeradjacent to side surfaces of the mask layer and to a top surface of then-doped region, and a second layer at least partially diffused into thesecond interfacial layer, to one or more of: reduce the thresholdvoltage of the transistor and reduce the thickness of an inversion layerof the semiconductor transistor device. The second layer is differentthan the first layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The drawings referenced herein form a part of the specification.Features shown in the drawing are meant as illustrative of only someembodiments of the invention, and not of all embodiments of theinvention, unless otherwise explicitly indicated, and implications tothe contrary are otherwise not to be made.

FIG. 1A is a flowchart of a first portion of a method for fabricating atransistor having replacement metal gates, according to an embodiment ofthe invention.

FIG. 1B is a flowchart of a second portion of a method for fabricating atransistor having replacement metal gates, according to an embodiment ofthe invention.

FIG. 2A is a diagram of a representative transistor after part 102 ofFIG. 1A has been performed, according to an embodiment of the invention.

FIG. 2B is a diagram of a representative transistor after parts 106,108, and 110 of FIG. 1A have been performed, according to an embodimentof the invention.

FIG. 2C is a diagram of a representative transistor after parts 112 and114 of FIG. 1A have been performed, according to an embodiment of theinvention.

FIG. 2D is a diagram of a representative transistor after parts 116,118, and 119 of FIG. 1A have been performed, according to an embodimentof the invention.

FIG. 2E is a diagram of a representative transistor after parts 126,128, 130, and 132 of FIG. 1B have been performed, according to anembodiment of the invention.

FIG. 2F is a diagram of a representative transistor after parts 140 and142 of FIG. 1B have been performed, according to an embodiment of theinvention.

DETAILED DESCRIPTION

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanying drawings that form apart hereof, and in which is shown by way of illustration specificexemplary embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilized,and logical, mechanical, and other changes may be made without departingfrom the spirit or scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the embodiment of the invention is defined only by theappended claims.

As stated in the background section, more recently transistors that havemetal gates instead of polysilicon gates have begun to be fabricated.Fabricating a transistor having a metal gate is generally more difficultthan fabricating a transistor having a polysilicon gate. Moreparticularly, fabricating a transistor having a replacement metal gateis more difficult than fabricating a transistor having a polysilicongate.

Embodiments of the invention are directed to fabricating a transistorhaving a replacement metal gate. Embodiments of the invention providefor such a fabrication process in which the threshold voltage of theresulting transistor and the thickness of the resulting transistor'sinversion layer are both reduced. As such, an integrated circuitemploying the inventive transistor consumes less power and operates at ahigher frequency than a comparable integrated circuit employing anexisting transistor.

FIGS. 1A and 1B show a first portion and a second portion, respectively,of a method 100 for fabricating a semiconductor transistor device,according to an embodiment of the invention. A substrate is providedwithin which there is a p-doped region and an n-doped region, over whichthere are polysilicon gates (102). The substrate itself may bepolysilicon as well in one embodiment. The gates are for the transistor.The substrate, including the doped regions and the polysilicon gatesover the doped region, may be formed conventionally. In one embodiment,there is further an etch stop layer between the gates and the substrate.

FIG. 2A shows a portion of a representative transistor 200 after part102 has been performed, according to an embodiment of the invention. Thetransistor 200 can also be referred to as a semiconductor transistordevice. The transistor 200 includes a substrate 202 and, within thesubstrate 202, a p-doped region 204 and an n-doped region 206. There isa polysilicon gate 208 over the p-doped region 204, and a polysilicongate 210 over the n-doped region 206. Furthermore, there is an etch stoplayer 211 between the polysilicon gates 208 and 210 and the substrate202. The etch stop 211 layer may be silicon oxide or silicon nitride inone embodiment.

Referring back to FIG. 1A, the polysilicon gates are removed from thesubstrate and a mask layer is formed over the substrate such that thedoped regions are exposed through corresponding holes within the masklayer (104). In one embodiment, part 104 may be performed as follows.Vertical spacers are formed on side surfaces of the polysilicon gates(106). The vertical spacers may be silicon nitride, silicon oxynitride,silicon oxide, or another material; however, the vertical spacers andthe etch stop layer have to be of different materials. The verticalspacers can be formed by depositing a layer over the substrate and thepolysilicon gates, and then etching the layer so that just the layer onthe side surfaces of the polysilicon gates remains.

A portion of the doped regions are replaced with silicide at each sideof the spacers (108). The purpose of the silicide is to lower thecontact resistance between the source and drain of the transistor and asubsequently formed contact. Part 108 can be etching the exposed dopedregions, and then depositing silicide in the spaces created. The masklayer is then formed over the substrate and over the polysilicon gates(110), and the transistor being formed is subjected tochemical-mechanical planarization (CMP) to remove the mask layer fromthe top surfaces of the polysilicon gates (112). The mask layer may besilicon oxide or nitride. The polysilicon gates and the etch stop layerare then removed (114).

FIG. 2B shows a portion of the transistor 200 after parts 106, 108, and110 have been performed, according to an embodiment of the invention.Vertical spacers 212 have been formed on the side surfaces of thepolysilicon gates 208 and 210. Portions of the p-doped region 204 andthe n-doped region 206 at sides of the spacers 212 have been replacedwith silicide 214. A mask layer 216, such as nitride, has been depositedover the substrate 202 and the polysilicon gates 210.

FIG. 2C shows a portion of the transistor 200 after parts 112 and 114have been performed, according to an embodiment of the invention. Thetransistor 200 has been subjected to CMP to expose the polysilicon gates210, and the polysilicon gates 210 have been removed, such by etchinguntil the etch stop layer 211 has been reached. Holes 218 and 220 areformed where the polysilicon gates 210 were. The hole 218 corresponds toand exposes the p-doped region 204 through the mask layer 216, whereasthe hole 220 corresponds to and exposes the n-doped region 206 throughthe mask layer 216. The etch stop layer 211 exposed within the holes 218and 220 is removed, because the layer 211 is no longer needed.

Referring back to FIG. 1A, the hole through which the n-doped region isexposed within the mask layer is covered with a first temporary layer(116). A first interfacial layer is then formed on the top and sidesurfaces of the mask layer, and on the top surface of the p-doped regionexposed through its corresponding hole (118). The interfacial layer maybe silicon oxide, silicon oxynitride, or another material. Theinterfacial layer can be formed during a substrate surface treatmentprocess, such as thermal oxidation, plasma nitridation, or deposition. Afirst layer is then deposited on the first interfacial layer (119).

The first layer can be deposited on the first interfacial layer in part119 in three different ways. First, a metal or metal oxide layer,followed by a high-k dielectric layer, can be deposited on the firstinterfacial layer. The metal or metal oxide layer and the high-k layertogether are considered the first layer that is deposited on the firstinterfacial layer. The metal or metal oxide layer may be lanthanum,lutetium, lanthanum oxide, or lutetium oxide, among other metals andamong other metal oxides, and the high-k dielectric layer may be hafniumoxide, titanium oxide, or zirconium oxide, among other high-kdielectrics.

Second, a high-k dielectric layer, followed by a metal or metal oxidelayer, can be deposited on the first interfacial layer. The high-kdielectric layer and the metal or metal oxide layer together areconsidered the first layer that is deposited on the first interfaciallayer. As before, the metal or metal oxide layer may be lanthanum,lutetium, lanthanum oxide, or lutetium oxide, among other metals andamong other metal oxides, and the high-k dielectric layer may be hafniumoxide, titanium oxide, or zirconium oxide, among other high-kdielectrics.

Third, a layer of a high-k dielectric material in which metal has beenmixed can be deposited on the first interfacial layer. As before, themetal may be lanthanum or lutetium, among other metals, and the high-kdielectric layer in which the metal is mixed may be hafnium oxide,titanium oxide, or zirconium oxide, among other high-k dielectrics. Inall three ways, the metal diffuses into the first interfacial layer.

The metal diffused into the first interfacial layer provides theresulting semiconductor transistor device with a lower thresholdvoltage, and with a thinner inversion layer. It has been found that bydepositing a metal layer onto the first interfacial layer, or by adepositing a metal oxide layer onto the first interfacial layer, themetal diffuses into the interfacial layer, and the resultingmetal-impregnated interfacial layer is what reduces the semiconductordevice's threshold voltage and inversion layer thickness.

FIG. 2D shows a portion of the transistor 200 after parts 116, 118, and119 have been performed, according to an embodiment of the invention.The hole 220 is covered with a first temporary layer 222, which may besilicon oxide. The layer 224 represents both the first interfacial layerand the first layer deposited on the first interfacial layer. Thus, thelayer 224 includes the interfacial layer having metal diffused therein,as well as the high-k dielectric layer.

Referring to FIG. 1B, the first temporary layer is removed (126), suchas by etching, and the hole through which the p-doped region is exposedwithin the mask layer is covered with a second temporary layer (128). Asecond interfacial layer is then deposited on the top and side surfacesof the mask layer, and on the top surface of the n-doped region exposedthrough its corresponding hole (130). The second interfacial layer maybe formed from the same material, and in the same way, as the firstinterfacial layer, as has been described above in relation to part 118.A second layer is then deposited on the second interfacial layer (132).

The second layer can be deposited on the second interfacial layer inpart 132 in any of the same three different ways that the first layercan be deposited on the first interfacial layer in part 119, as has beendescribed above. As such, first, a metal or metal oxide layer, followedby a high-k dielectric layer, can be deposited on the second interfaciallayer. The metal or metal oxide layer and the high-k layer together areconsidered the second layer that is deposited on the second interfacialsecond. The metal or metal oxide layer may be aluminum or aluminumoxide, among other metals and among other metal oxides, and the high-kdielectric layer may be hafnium oxide, titanium oxide, or zirconiumoxide, among other high-k dielectrics.

Second, a high-k dielectric layer, followed by a metal or metal oxidelayer, can be deposited on the second interfacial layer. The high-kdielectric layer and the metal or metal oxide layer together areconsidered the second layer that is deposited on the second interfaciallayer. As before, the metal or metal oxide layer may be aluminum oraluminum oxide, among other metals and among other metal oxides, and thehigh-k dielectric layer may be hafnium oxide, titanium oxide, orzirconium oxide, among other high-k dielectrics.

Third, a layer of a high-k dielectric material in which metal has beenmixed can be deposited on the second interfacial layer. As before, themetal may be aluminum, among other metals, and the high-k dielectriclayer in which the metal is mixed may be hafnium oxide, titanium oxide,or zirconium oxide, among other high-k dielectrics. In all three ways,the metal diffuses into the second interfacial layer.

It is noted that the metal that is part of the second layer deposited onthe second interfacial layer in part 132 is different than the metalthat is part of the first layer deposited on the first interfacial layerin part 119. For example, the metal deposited on the first interfaciallayer over the p-doped region may be lanthanum or lutetium, whereas themetal deposited on the second interfacial layer over the n-doped regionmay be aluminum. This is because that the n-field effect transistor(n-FET) and the p-FET have to have opposite charge polarities to reducetheir threshold voltages.

FIG. 2E shows a portion of the transistor 200 after parts 126, 128, 130,and 132 have been performed, according to an embodiment of theinvention. The first temporary layer 222 has been removed, and the hole218 has been covered with a second temporary layer 226, which may besilicon oxide. The layer 228 represents both the second interfaciallayer deposited in part 130 and the second layer deposited on the secondinterfacial layer in part 132. Thus, the layer 228 includes theinterfacial layer having metal diffused therein, as well as the high-kdielectric layer.

Referring back to FIG. 1B, the second temporary is removed (140), and aconductive plug is formed within the hole over each doped region (142).In one embodiment, the conductive plug is formed within each hole bydepositing a work function metal within each hole (144), and thendepositing metal, such as aluminum, over the work function metal (146).Reactive-ion etching, or another type of etching, may be performedbetween parts 144 and 146. The semiconductor transistor device may beplanarized after part 146, such as by chemical-mechanical polishing. Thework function metal may be titanium nitride, and the metal depositedover the work function metal may be aluminum. The purpose of the workfunction metal is to adjust the threshold voltages for the n-FET and thep-FET. The threshold voltage adjustment resulting from the work functionmetal is additive to the threshold voltage adjustment resulting from themetal diffusion within the interfacial layers.

It is noted that the conductive plugs, together with the interfaciallayers and the first and the second layers deposited on the interfaciallayers, make up the replacement gates of the semiconductor transistordevice formed by the method 100. The presence of the metal diffusedwithin the interfacial layers, as a result of depositing the first andthe second layers on the interfacial layers, imbues the semiconductortransistor device with certain properties, as has been described above.These properties are namely a reduced threshold voltage, and a reducedinversion layer thickness.

FIG. 2F shows the transistor 200 after parts 140 and 142 have beenperformed, according to an embodiment of the invention. The secondtemporary layer 226 has been removed. A conductive plug 230 has beenformed within the hole 218 over the p-doped region 204. Likewise, aconductive plug 232 has been formed within the hole 220 over the n-dopedregion 206.

The method 100 has been described such that a semiconductor transistordevice that has two metal replacement gates is fabricated. However, inother embodiments of the invention, a semiconductor transistor that hasjust one replacement gate may be fabricated. That is, instead of havingtwo doped regions over which there are replacement gates, there may bejust one doped region over which there is a replacement gate.

Finally, it is noted that, although specific embodiments have beenillustrated and described herein, it will be appreciated by those ofordinary skill in the art that any arrangement calculated to achieve thesame purpose may be substituted for the specific embodiments shown. Thisapplication is thus intended to cover any adaptations or variations ofembodiments of the present invention. As such and therefore, it ismanifestly intended that this invention be limited only by the claimsand equivalents thereof.

We claim:
 1. A method for fabricating a transistor, comprising: forminga mask layer over a substrate; removing a polysilicon gate over a dopedregion of the substrate such that the doped region is exposed through ahole within the mask layer; covering the hole through which the dopedregion is exposed within the mask layer with a temporary layer; whilethe hole through which the doped region is exposed within the mask layeris covered with the temporary layer, depositing an interfacial layer ontop and side surfaces of the mask layer and on a top surface of thedoped region exposed through the hole, and depositing a layer on theinterfacial layer adapted to one or more of: reducing a thresholdvoltage of the transistor and reducing a thickness of an inversion layerof the transistor; removing the temporary layer; forming a conductiveplug within the hole of the mask layer, wherein the interfacial layer,the layer on the interfacial layer, and the conductive plug together area replacement gate of the transistor.
 2. The method of claim 1, whereindepositing the layer on the interfacial layer comprises: depositing ametal layer or a metal oxide layer on the interfacial layer, the metalfrom the metal layer or the metal oxide layer to diffuse into theinterfacial layer; and depositing a high-k dielectric layer on the metallayer or the metal oxide layer.
 3. The method of claim 1, whereindepositing the layer on the interfacial layer comprises: depositing ahigh-k dielectric layer on the interfacial layer; and depositing a metallayer or a metal oxide layer on the high-k dielectric layer, the metalfrom the metal layer or the metal oxide layer to diffuse into theinterfacial layer.
 4. The method of claim 1, wherein depositing thelayer on the interfacial layer comprises: depositing a high-k dielectriclayer in which metal has been mixed, on the interfacial layer, whereinthe metal is to diffuse into the interfacial layer.
 5. The method ofclaim 1, wherein depositing the layer on the interfacial layer resultsin a metal diffusing into the interfacial layer, the metal comprisingone of lanthanum, lutetium, and aluminum.
 6. The method of claim 1,wherein forming the conductive plug within the hole of the mask layercomprises: depositing a work function metal within the hole of the masklayer; and depositing a second metal over the work function metal withinthe hole of the mask layer.
 7. A method for forming a transistor,comprising: forming vertical spacers on side surfaces of a polysilicongate over a doped region of a substrate; replacing a portion of thedoped region at a side of each vertical spacer beginning at the topsurface of the doped region with silicide prior to removal of thepolysilicon gate such that the silicide does not replace any portion ofthe doped region directly under the polysilicon gate, the silicidehaving a top surface flush with a top surface of the doped region of thesubstrate, the silicide lowering contact resistance between a source anda drain of the transistor and a contact of the transistor; forming amask layer over the substrate; removing the polysilicon gate such thatthe doped region is exposed through a hole within the mask layer;covering the hole through which the doped region is exposed within themask layer with a temporary layer; while the hole through which thedoped region is exposed within the mask layer is covered with thetemporary layer, depositing an interfacial layer on top and sidesurfaces of the mask layer and on a top surface of the doped regionexposed through the hole, and depositing a layer on the interfaciallayer adapted to one or more of: reducing a threshold voltage of thetransistor and reducing a thickness of an inversion layer of thetransistor; depositing a work function metal within the hole of the masklayer; removing the temporary layer; depositing a second metal over thework function metal within the hole of the mask layer, the work functionmetal and the second metal being a conductive plug, wherein theinterfacial layer, the layer on the interfacial layer, and theconductive plug together are a replacement gate of the transistor. 8.The method of claim 7, wherein depositing the layer on the interfaciallayer comprises: depositing a metal layer or a metal oxide layer on theinterfacial layer, the metal from the metal layer or the metal oxidelayer to diffuse into the interfacial layer; and depositing a high-kdielectric layer on the metal layer or the metal oxide layer.
 9. Themethod of claim 7, wherein depositing the layer on the interfacial layercomprises: depositing a high-k dielectric layer on the interfaciallayer; and depositing a metal layer or a metal oxide layer on the high-kdielectric layer, such that the metal from the metal layer or the metaloxide layer diffuses into the interfacial layer.
 10. The method of claim7, wherein depositing the layer on the interfacial layer comprises:depositing a high-k dielectric layer in which metal has been mixed, onthe interfacial layer, such that the metal diffuses into the interfaciallayer.
 11. The method of claim 7, wherein depositing the layer on theinterfacial layer results in a metal diffusing into the interfaciallayer, the metal comprising one of lanthanum, lutetium, and aluminum.12. A semiconductor transistor device comprising: a substrate; a dopedregion within and exposed through the substrate, the doped region havinga first top surface and a center thereof, and a second top surface atsides thereof, the second top surface lower than the first top surfaceto define notches within the doped region between the first top surfaceand the second top surface; vertical spacers over the doped region;silicide within the notches defined within the doped region, thesilicide having a top surface flush with the first top surface of thedoped region of the substrate, the silicide lowering contact resistancebetween a source and a drain of the transistor and a contact of thetransistor; a mask layer over the substrate and having a hole throughwhich the doped region is exposed through the mask layer; and areplacement metal gate within the mask layer and over the doped region,wherein the replacement metal gate comprises: a conductive plug withinthe hole of the mask layer; an interfacial layer adjacent to sidesurfaces of the mask layer and to a top surface of the doped region; anda layer at least partially diffused into the interfacial layer, to oneor more of: reduce a threshold voltage of the transistor and reduce athickness of an inversion layer of the semiconductor transistor device.13. The semiconductor transistor device of claim 12, wherein the layercomprises: a metal layer or a metal oxide layer; and a high-k dielectriclayer.
 14. The semiconductor transistor device of claim 13, wherein themetal layer is one of an aluminum layer, a lutetium layer, and alanthanum layer, the metal oxide layer is one of an aluminum oxidelayer, a lutetium oxide layer, and a lanthanum oxide layer, and thehigh-k dielectric layer is one of a hafnium oxide layer, a titaniumoxide layer, and a zirconium oxide layer.
 15. The semiconductortransistor device of claim 12, wherein the layer comprises a high-kdielectric material into which a metal has been mixed.
 16. Thesemiconductor transistor device of claim 15, wherein the metal is one ofaluminum, lutetium, and lanthanum, and the high-k dielectric material isone of a hafnium oxide, titanium oxide, and zirconium oxide.